Purpose: Current patient-specific quality assurance (QA) methodologies adopted by the medical physics community are well established, but lack in guidance on the utilization of portal dosimetry (PD), as it continues to grow in popularity. The focus of this study was to clinically implement PD, compare the patient-specific QA analysis features to those employed by SunNuclear (SNC) MapCHECK2, and design a more robust QA program which utilizes PD to improve the efficiency of faculty resources and accuracy of complex dose distribution analyses.
Methods: Implementation of Varian Portal Dosimetry requires 4 sets of data: (I) diagonal profiles at dmax for largest field size; (II) Varian 5 bar fluence pattern; (III) dose normalization of the flat panel detector (FPD); (IV) measured output factors of the FPD. These data characterize the dosimetric effects of the MLC and configure the dose deposition kernel used to compute the fluence deposited on the FPD. Patient-specific QA was measured and analyzed using both PD and MapCHECK2 methods. The treatment fields assessed were comprised of clinically relevant sites associated with each beam energy (6MV, 10MV, and 16MV). SNC Patient software was used for gamma analysis of the MapCHECK2 measurements and ARIA Portal Dosimetry was used for gamma analysis of the PD measurements.
Results: The average pass-rates across the treatment fields using a dose difference (DD) of 3% and distance-to-agreement (DTA) of 3mm was 95.2% and 97.7% for PD and MapCHECK2 respectively. For a DD of 2% and DTA of 2mm, the rates were 90.7% and 95.2%. All fields which did not pass using PD gamma analysis, passed when using SNC gamma analysis.
Conclusion: Portal dosimetry shows promise as an improved method of patient-specific QA as it requires less faculty resources, contains increased sensitivity within its employment of gamma analysis, and has an intrinsic improvement in measurement spatial resolution.